declares that subtraction is to be implemented by method
"minus()" in the class
"Number" (or one of its base classes), and
that the function "Number::muas()" is to
be used for the assignment form of multiplication,
"*=". It also defines an anonymous
subroutine to implement stringification: this is called whenever an object
blessed into the package "Number" is used
in a string context (this subroutine might, for example, return the number
as a Roman numeral).

Calling Conventions and Magic Autogeneration

The following sample implementation of
"minus()" (which assumes that
"Number" objects are simply blessed
references to scalars) illustrates the calling conventions:

Three arguments are passed to all subroutines specified in the
"use overload" directive (with exceptions
- see below, particularly "nomethod").

The first of these is the operand providing the overloaded
operator implementation - in this case, the object whose
"minus()" method is being called.

The second argument is the other operand, or
"undef" in the case of a unary
operator.

The third argument is set to TRUE if (and only if) the two
operands have been swapped. Perl may do this to ensure that the first
argument ($self) is an object implementing the
overloaded operation, in line with general object calling conventions. For
example, if $x and $y are
"Number"s:

Perl may also use "minus()" to
implement other operators which have not been specified in the
"use overload" directive, according to the
rules for "Magic Autogeneration" described later. For example, the
"use overload" above declared no
subroutine for any of the operators "--",
"neg" (the overload key for unary minus),
or "-=". Thus

Note the "undef"s: where
autogeneration results in the method for a standard operator which does not
change either of its operands, such as
"-", being used to implement an operator
which changes the operand ("mutators": here,
"--" and
"-="), Perl passes undef as the third
argument. This still evaluates as FALSE, consistent with the fact that the
operands have not been swapped, but gives the subroutine a chance to alter
its behaviour in these cases.

In all the above examples,
"minus()" is required only to return the
result of the subtraction: Perl takes care of the assignment to
$x. In fact, such methods should not modify
their operands, even if "undef" is passed
as the third argument (see "Overloadable Operations").

The same is not true of implementations of
"++" and
"--": these are expected to modify their
operand. An appropriate implementation of
"--" might look like

use overload '--' => "decr",
# ...
sub decr { --${$_[0]}; }

If the "bitwise" feature is enabled (see feature), a
fifth TRUE argument is passed to subroutines handling
"&",
"|", "^"
and "~". This indicates that the caller is
expecting numeric behaviour. The fourth argument will be
"undef", as that position
($_[3]) is reserved for use by
"nomethod".

Mathemagic, Mutators, and Copy Constructors

The term 'mathemagic' describes the overloaded implementation of
mathematical operators. Mathemagical operations raise an issue. Consider the
code:

$a = $b;
--$a;

If $a and $b are
scalars then after these statements

$a == $b - 1

An object, however, is a reference to blessed data, so if
$a and $b are objects then
the assignment "$a = $b" copies only the
reference, leaving $a and $b
referring to the same object data. One might therefore expect the operation
"--$a" to decrement
$b as well as $a. However,
this would not be consistent with how we expect the mathematical operators
to work.

Perl resolves this dilemma by transparently calling a copy
constructor before calling a method defined to implement a mutator
("--",
"+=", and so on.). In the above example,
when Perl reaches the decrement statement, it makes a copy of the object
data in $a and assigns to $a
a reference to the copied data. Only then does it call
"decr()", which alters the copied data,
leaving $b unchanged. Thus the object metaphor is
preserved as far as possible, while mathemagical operations still work
according to the arithmetic metaphor.

Note: the preceding paragraph describes what happens when Perl
autogenerates the copy constructor for an object based on a scalar. For
other cases, see "Copy Constructor".

Most of the overloadable operators map one-to-one to these keys.
Exceptions, including additional overloadable operations not apparent from
this hash, are included in the notes which follow. This list is subject to
growth over time.

A warning is issued if an attempt is made to register an operator
not found above.

"not"

The operator "not" is not a
valid key for "use overload". However,
if the operator "!" is overloaded then
the same implementation will be used for
"not" (since the two operators differ
only in precedence).

"neg"

The key "neg" is used for
unary minus to disambiguate it from binary
"-".

"++",
"--"

Assuming they are to behave analogously to Perl's
"++" and
"--", overloaded implementations of
these operators are required to mutate their operands.

No distinction is made between prefix and postfix forms of the
increment and decrement operators: these differ only in the point at
which Perl calls the associated subroutine when evaluating an
expression.

Assignments

+= -= *= /= %= **= <<= >>= x= .=
&= |= ^= &.= |.= ^.=

Simple assignment is not overloadable (the
'=' key is used for the "Copy
Constructor"). Perl does have a way to make assignments to an
object do whatever you want, but this involves using tie(), not
overload - see "tie" in perlfunc and the "COOKBOOK"
examples below.

The subroutine for the assignment variant of an operator is
required only to return the result of the operation. It is permitted to
change the value of its operand (this is safe because Perl calls the
copy constructor first), but this is optional since Perl assigns the
returned value to the left-hand operand anyway.

An object that overloads an assignment operator does so only
in respect of assignments to that object. In other words, Perl never
calls the corresponding methods with the third argument (the
"swap" argument) set to TRUE. For example, the operation

$a *= $b

cannot lead to $b's implementation of
"*=" being called, even if
$a is a scalar. (It can, however, generate a
call to $b's method for
"*").

Non-mutators with a mutator variant

+ - * / % ** << >> x .
& | ^ &. |. ^.

As described above, Perl may call methods for operators like
"+" and
"&" in the course of implementing
missing operations like "++",
"+=", and
"&=". While these methods may
detect this usage by testing the definedness of the third argument, they
should in all cases avoid changing their operands. This is because Perl
does not call the copy constructor before invoking these methods.

"int"

Traditionally, the Perl function
"int" rounds to 0 (see "int"
in perlfunc), and so for floating-point-like types one should follow the
same semantic.

String, numeric, boolean, and regexp conversions

"" 0+ bool

These conversions are invoked according to context as
necessary. For example, the subroutine for
'""' (stringify) may be used where the
overloaded object is passed as an argument to
"print", and that for
'bool' where it is tested in the condition of a
flow control statement (like "while")
or the ternary "?:" operation.

Of course, in contexts like, for example,
"$obj + 1", Perl will invoke
$obj's implementation of
"+" rather than (in this example)
converting $obj to a number using the numify
method '0+' (an exception to this is when no
method has been provided for '+' and
"fallback" is set to TRUE).

The subroutines for '""',
'0+', and 'bool' can
return any arbitrary Perl value. If the corresponding operation for this
value is overloaded too, the operation will be called again with this
value.

As a special case if the overload returns the object itself
then it will be used directly. An overloaded conversion returning the
object is probably a bug, because you're likely to get something that
looks like
"YourPackage=HASH(0x8172b34)".

qr

The subroutine for 'qr' is used
wherever the object is interpolated into or used as a regexp, including
when it appears on the RHS of a "=~"
or "!~" operator.

"qr" must return a compiled
regexp, or a ref to a compiled regexp (such as
"qr//" returns), and any further
overloading on the return value will be ignored.

Iteration

If "<>" is overloaded
then the same implementation is used for both the read-filehandle
syntax "<$var>" and
globbing syntax
"<${var}>".

File tests

The key '-X' is used to specify a
subroutine to handle all the filetest operators
("-f",
"-x", and so on: see "-X" in
perlfunc for the full list); it is not possible to overload any filetest
operator individually. To distinguish them, the letter following the '-'
is passed as the second argument (that is, in the slot that for binary
operators is used to pass the second operand).

Calling an overloaded filetest operator does not affect the
stat value associated with the special filehandle
"_". It still refers to the result of
the last "stat",
"lstat" or unoverloaded filetest.

This overload was introduced in Perl 5.12.

Matching

The key "~~" allows you to
override the smart matching logic used by the
"~~" operator and the switch construct
("given"/"when").
See "Switch Statements" in perlsyn and feature.

Unusually, the overloaded implementation of the smart match
operator does not get full control of the smart match behaviour. In
particular, in the following code:

rather, the smart match distributive rule takes precedence, so
$obj is smart matched against each array element
in turn until a match is found, so you may see between one and three of
these calls instead:

$obj->match(1,0);
$obj->match(2,0);
$obj->match(3,0);

Consult the match table in "Smartmatch Operator" in
perlop for details of when overloading is invoked.

Dereferencing

${} @{} %{} &{} *{}

If these operators are not explicitly overloaded then they
work in the normal way, yielding the underlying scalar, array, or
whatever stores the object data (or the appropriate error message if the
dereference operator doesn't match it). Defining a catch-all
'nomethod' (see below) makes no difference to
this as the catch-all function will not be called to implement a missing
dereference operator.

If a dereference operator is overloaded then it must return a
reference of the appropriate type (for example, the subroutine
for key '${}' should return a reference to a
scalar, not a scalar), or another object which overloads the operator:
that is, the subroutine only determines what is dereferenced and the
actual dereferencing is left to Perl. As a special case, if the
subroutine returns the object itself then it will not be called again -
avoiding infinite recursion.

If a method for an operation is not found then Perl tries to autogenerate a
substitute implementation from the operations that have been defined.

Note: the behaviour described in this section can be disabled by
setting "fallback" to FALSE (see
"fallback").

In the following tables, numbers indicate priority. For example,
the table below states that, if no implementation for
'!' has been defined then Perl will implement it
using 'bool' (that is, by inverting the value
returned by the method for 'bool'); if boolean
conversion is also unimplemented then Perl will use
'0+' or, failing that,
'""'.

Note: The iterator ('<>') and file
test ('-X') operators work as normal: if the operand
is not a blessed glob or IO reference then it is converted to a string
(using the method for '""',
'0+', or 'bool') to be
interpreted as a glob or filename.

Note also that the copy constructor (key
'=') may be autogenerated, but only for objects
based on scalars. See "Copy Constructor".

Minimal Set of Overloaded Operations

Since some operations can be automatically generated from others,
there is a minimal set of operations that need to be overloaded in order to
have the complete set of overloaded operations at one's disposal. Of course,
the autogenerated operations may not do exactly what the user expects. The
minimal set is:

The 'nomethod' key is used to specify a
catch-all function to be called for any operator that is not individually
overloaded. The specified function will be passed four parameters. The first
three arguments coincide with those that would have been passed to the
corresponding method if it had been defined. The fourth argument is the
"use overload" key for that missing
method. If the "bitwise" feature is enabled (see feature), a fifth
TRUE argument is passed to subroutines handling
"&",
"|", "^"
and "~" to indicate that the caller is
expecting numeric behaviour.

For example, if $a is an object blessed
into a package declaring

use overload 'nomethod' => 'catch_all', # ...

then the operation

3 + $a

could (unless a method is specifically declared for the key
'+') result in a call

catch_all($a, 3, 1, '+')

See "How Perl Chooses an Operator Implementation".

"fallback"

The value assigned to the key 'fallback'
tells Perl how hard it should try to find an alternative way to implement a
missing operator.

defined, but FALSE

use overload "fallback" => 0, # ... ;

This disables "Magic Autogeneration".

"undef"

In the default case where no value is explicitly assigned to
"fallback", magic autogeneration is
enabled.

TRUE

The same as for "undef", but
if a missing operator cannot be autogenerated then, instead of issuing
an error message, Perl is allowed to revert to what it would have done
for that operator if there had been no "use
overload" directive.

Note: in most cases, particularly the "Copy
Constructor", this is unlikely to be appropriate behaviour.

See "How Perl Chooses an Operator Implementation".

Copy Constructor

As mentioned above, this operation is called when a mutator is
applied to a reference that shares its object with some other reference. For
example, if $b is mathemagical, and
'++' is overloaded with
'incr', and '=' is
overloaded with 'clone', then the code

$a = $b;
# ... (other code which does not modify $a or $b) ...
++$b;

would be executed in a manner equivalent to

$a = $b;
# ...
$b = $b->clone(undef, "");
$b->incr(undef, "");

Note:

The subroutine for '=' does not overload the Perl
assignment operator: it is used only to allow mutators to work as
described here. (See "Assignments" above.)

As for other operations, the subroutine implementing '=' is passed three
arguments, though the last two are always
"undef" and
''.

The copy constructor is called only before a call to a function declared
to implement a mutator, for example, if
"++$b;" in the code above is effected
via a method declared for key '++' (or 'nomethod',
passed '++' as the fourth argument) or, by
autogeneration, '+='. It is not called if the
increment operation is effected by a call to the method for
'+' since, in the equivalent code,

$a = $b;
$b = $b + 1;

the data referred to by $a is
unchanged by the assignment to $b of a reference
to new object data.

The copy constructor is not called if Perl determines that it is
unnecessary because there is no other reference to the data being
modified.

If 'fallback' is undefined or TRUE then a copy
constructor can be autogenerated, but only for objects based on scalars.
In other cases it needs to be defined explicitly. Where an object's data
is stored as, for example, an array of scalars, the following might be
appropriate:

use overload '=' => sub { bless [ @{$_[0]} ] }, # ...

If 'fallback' is TRUE and no copy constructor is
defined then, for objects not based on scalars, Perl may silently fall
back on simple assignment - that is, assignment of the object reference.
In effect, this disables the copy constructor mechanism since no new copy
of the object data is created. This is almost certainly not what you want.
(It is, however, consistent: for example, Perl's fallback for the
"++" operator is to increment the
reference itself.)

Which is checked first, "nomethod" or
"fallback"? If the two operands of an
operator are of different types and both overload the operator, which
implementation is used? The following are the precedence rules:

1.

If the first operand has declared a subroutine to overload the operator
then use that implementation.

2.

Otherwise, if fallback is TRUE or undefined for the first operand then see
if the rules for autogeneration allows another of its operators to be used
instead.

3.

Unless the operator is an assignment
("+=",
"-=", etc.), repeat step (1) in respect
of the second operand.

4.

Repeat Step (2) in respect of the second operand.

5.

If the first operand has a "nomethod" method then use that.

6.

If the second operand has a "nomethod" method then use
that.

7.

If "fallback" is TRUE for both operands
then perform the usual operation for the operator, treating the operands
as numbers, strings, or booleans as appropriate for the operator (see
note).

8.

Nothing worked - die.

Where there is only one operand (or only one operand with
overloading) the checks in respect of the other operand above are
skipped.

There are exceptions to the above rules for dereference operations
(which, if Step 1 fails, always fall back to the normal, built-in
implementations - see Dereferencing), and for
"~~" (which has its own set of rules - see
"Matching" under "Overloadable
Operations" above).

Note on Step 7: some operators have a different semantic depending
on the type of their operands. As there is no way to instruct Perl to treat
the operands as, e.g., numbers instead of strings, the result here may not
be what you expect. See "BUGS AND PITFALLS".

The restriction for the comparison operation is that even if, for example,
"cmp" should return a blessed reference, the
autogenerated "lt" function will produce
only a standard logical value based on the numerical value of the result of
"cmp". In particular, a working numeric
conversion is needed in this case (possibly expressed in terms of other
conversions).

Similarly, ".=" and
"x=" operators lose their mathemagical
properties if the string conversion substitution is applied.

When you chop() a mathemagical object it is promoted to a
string and its mathemagical properties are lost. The same can happen with
other operations as well.

is a string, it is interpreted as a method name - which may
(in the usual way) be inherited from another class.

Overloading of an operation is inherited by derived classes

Any class derived from an overloaded class is also overloaded and inherits
its operator implementations. If the same operator is overloaded in more
than one ancestor then the implementation is determined by the usual
inheritance rules.

For example, if "A" inherits
from "B" and
"C" (in that order),
"B" overloads
"+" with
"\&D::plus_sub", and
"C" overloads
"+" by
"plus_meth", then the subroutine
"D::plus_sub" will be called to
implement operation "+" for an object
in package "A".

Note that in Perl version prior to 5.18 inheritance of the
"fallback" key was not governed by the
above rules. The value of "fallback" in
the first overloaded ancestor was used. This was fixed in 5.18 to follow the
usual rules of inheritance.

Gives the string value of "arg" as in
the absence of stringify overloading. If you are using this to get the
address of a reference (useful for checking if two references point to the
same thing) then you may be better off using
"Scalar::Util::refaddr()", which is
faster.

For some applications, the Perl parser mangles constants too much. It is
possible to hook into this process via
"overload::constant()" and
"overload::remove_constant()" functions.

These functions take a hash as an argument. The recognized keys of
this hash are:

integer

to overload integer constants,

float

to overload floating point constants,

binary

to overload octal and hexadecimal constants,

q

to overload "q"-quoted strings, constant
pieces of "qq"- and
"qx"-quoted strings and
here-documents,

qr

to overload constant pieces of regular expressions.

The corresponding values are references to functions which take
three arguments: the first one is the initial string form of the
constant, the second one is how Perl interprets this constant, the third one
is how the constant is used. Note that the initial string form does not
contain string delimiters, and has backslashes in backslash-delimiter
combinations stripped (thus the value of delimiter is not relevant for
processing of this string). The return value of this function is how this
constant is going to be interpreted by Perl. The third argument is undefined
unless for overloaded "q"- and
"qr"- constants, it is
"q" in single-quote context (comes from
strings, regular expressions, and single-quote HERE documents), it is
"tr" for arguments of
"tr"/"y"
operators, it is "s" for right-hand side
of "s"-operator, and it is
"qq" otherwise.

Since an expression "ab$cd,," is
just a shortcut for 'ab' . $cd . ',,', it is
expected that overloaded constant strings are equipped with reasonable
overloaded catenation operator, otherwise absurd results will result.
Similarly, negative numbers are considered as negations of positive
constants.

Note that it is probably meaningless to call the functions
overload::constant() and overload::remove_constant() from
anywhere but import() and unimport() methods. From these
methods they may be called as

The table of methods for all operations is cached in magic for the
symbol table hash for the package. The cache is invalidated during
processing of "use overload",
"no overload", new function definitions,
and changes in @ISA.

(Every SVish thing has a magic queue, and magic is an entry in
that queue. This is how a single variable may participate in multiple forms
of magic simultaneously. For instance, environment variables regularly have
two forms at once: their %ENV magic and their taint
magic. However, the magic which implements overloading is applied to the
stashes, which are rarely used directly, thus should not slow down
Perl.)

If a package uses overload, it carries a special flag. This flag
is also set when new functions are defined or @ISA
is modified. There will be a slight speed penalty on the very first
operation thereafter that supports overloading, while the overload tables
are updated. If there is no overloading present, the flag is turned off.
Thus the only speed penalty thereafter is the checking of this flag.

It is expected that arguments to methods that are not explicitly
supposed to be changed are constant (but this is not enforced).

Note several important features of this example. First of all, the
actual type of $bar is a scalar reference,
and we do not overload the scalar dereference. Thus we can get the
actual non-overloaded contents of $bar by
just using $$bar (what we do in functions which
overload dereference). Similarly, the object returned by the
TIEHASH() method is a scalar reference.

Second, we create a new tied hash each time the hash syntax is
used. This allows us not to worry about a possibility of a reference loop,
which would lead to a memory leak.

Both these problems can be cured. Say, if we want to overload hash
dereference on a reference to an object which is implemented as a
hash itself, the only problem one has to circumvent is how to access this
actual hash (as opposed to the virtual hash exhibited by the
overloaded dereference operator). Here is one possible fetching routine:

Now if $baz is overloaded like this, then
$baz is a reference to a reference to the
intermediate array, which keeps a reference to an actual array, and the
access hash. The tie()ing object for the access hash is a reference
to a reference to the actual array, so

There are no loops of references.

Both "objects" which are blessed into the class
"two_refs1" are references to a
reference to an array, thus references to a scalar. Thus the
accessor expression "$$foo->[$ind]"
involves no overloaded operations.

This module is very unusual as overloaded modules go: it does not
provide any usual overloaded operators, instead it provides an
implementation for ""nomethod"".
In this example the "nomethod" subroutine
returns an object which encapsulates operations done over the objects:
"symbolic->new(3)" contains
"['n', 3]", "2
+symbolic->new(3)" contains
"['+', 2, ['n', 3]]".

Here is an example of the script which "calculates" the
side of circumscribed octagon using the above package:

Note that while we obtained this value using a nice little script,
there is no simple way to use this value. In fact this value may be
inspected in debugger (see perldebug), but only if
"bareStringify"Option is set, and
not via "p" command.

If one attempts to print this value, then the overloaded operator
"" will be called, which will call
"nomethod" operator. The result of this
operator will be stringified again, but this result is again of type
"symbolic", which will lead to an infinite
loop.

The method "pretty" is doing
object-to-string conversion, so it is natural to overload the operator
"" using this method. However, inside such
a method it is not necessary to pretty-print the components$a and $b of an object. In
the above subroutine "[$meth $a $b]" is a
catenation of some strings and components $a and
$b. If these components use overloading, the
catenation operator will look for an overloaded operator
"."; if not present, it will look for an
overloaded operator "". Thus it is enough
to use

and one can inspect the value in debugger using all the possible
methods.

Something is still amiss: consider the loop variable
$cnt of the script. It was a number, not an object.
We cannot make this value of type
"symbolic", since then the loop will not
terminate.

Indeed, to terminate the cycle, the $cnt
should become false. However, the operator
"bool" for checking falsity is overloaded
(this time via overloaded ""), and returns
a long string, thus any object of type
"symbolic" is true. To overcome this, we
need a way to compare an object to 0. In fact, it is easier to write a
numeric conversion routine.

Here is the text of symbolic.pm with such a routine added
(and slightly modified str()):

All the work of numeric conversion is done in
%subr and num(). Of course,
%subr is not complete, it contains only operators
used in the example below. Here is the extra-credit question: why do we need
an explicit recursion in num()? (Answer is at the end of this
section.)

The above module is very primitive. It does not implement mutator
methods ("++",
"-=" and so on), does not do deep copying
(not required without mutators!), and implements only those arithmetic
operations which are used in the example.

To implement most arithmetic operations is easy; one should just
use the tables of operations, and change the code which fills
%subr to

Since subroutines implementing assignment operators are not
required to modify their operands (see "Overloadable Operations"
above), we do not need anything special to make
"+=" and friends work, besides adding
these operators to %subr and defining a copy
constructor (needed since Perl has no way to know that the implementation of
'+=' does not mutate the argument - see "Copy
Constructor").

This finishes implementation of a primitive symbolic calculator in
50 lines of Perl code. Since the numeric values of subexpressions are not
cached, the calculator is very slow.

Here is the answer for the exercise: In the case of str(),
we need no explicit recursion since the overloaded
"."-operator will fall back to an existing
overloaded operator "". Overloaded
arithmetic operators do not fall back to numeric conversion if
"fallback" is not explicitly requested.
Thus without an explicit recursion num() would convert
"['+', $a, $b]" to
"$a + $b", which would just rebuild the
argument of num().

If you wonder why defaults for conversion are different for
str() and num(), note how easy it was to write the symbolic
calculator. This simplicity is due to an appropriate choice of defaults. One
extra note: due to the explicit recursion num() is more fragile than
sym(): we need to explicitly check for the type of
$a and $b. If components
$a and $b happen to be of
some related type, this may lead to problems.

When Perl is run with the -Do switch or its equivalent, overloading
induces diagnostic messages.

Using the "m" command of Perl
debugger (see perldebug) one can deduce which operations are overloaded (and
which ancestor triggers this overloading). Say, if
"eq" is overloaded, then the method
"(eq" is shown by debugger. The method
"()" corresponds to the
"fallback" key (in fact a presence of this
method shows that this package has overloading enabled, and it is what is
used by the "Overloaded" function of
module "overload").

The module might issue the following warnings:

Odd number of arguments for overload::constant

(W) The call to overload::constant contained an odd number of arguments.
The arguments should come in pairs.

'%s' is not an overloadable type

(W) You tried to overload a constant type the overload package is unaware
of.

'%s' is not a code reference

(W) The second (fourth, sixth, ...) argument of overload::constant needs
to be a code reference. Either an anonymous subroutine, or a reference to
a subroutine.

overload arg '%s' is invalid

(W) "use overload" was passed an
argument it did not recognize. Did you mistype an operator?

You might expect this to output "12". In fact, it
prints "<": the ASCII result of treating "|" as a
bitwise string operator - that is, the result of treating the operands
as the strings "4" and "8" rather than numbers. The
fact that numify ("0+") is implemented
but stringify ("") isn't makes no
difference since the latter is simply autogenerated from the former.

The only way to change this is to provide your own subroutine
for '|'.

Magic autogeneration increases the potential for inadvertently creating
self-referential structures. Currently Perl will not free self-referential
structures until cycles are explicitly broken. For example,

use overload '+' => 'add';
sub add { bless [ \$_[0], \$_[1] ] };

is asking for trouble, since

$obj += $y;

will effectively become

$obj = add($obj, $y, undef);

with the same result as

$obj = [\$obj, \$foo];

Even if no explicit assignment-variants of operators
are present in the script, they may be generated by the optimizer. For
example,

"obj = $obj\n"

may be optimized to

my $tmp = 'obj = ' . $obj; $tmp .= "\n";

The symbol table is filled with names looking like line-noise.

This bug was fixed in Perl 5.18, but may still trip you up if you are
using older versions:

For the purpose of inheritance every overloaded package
behaves as if "fallback" is present
(possibly undefined). This may create interesting effects if some
package is not overloaded, but inherits from two overloaded
packages.

Before Perl 5.14, the relation between overloading and tie()ing was
broken. Overloading was triggered or not based on the previous
class of the tie()d variable.

This happened because the presence of overloading was checked
too early, before any tie()d access was attempted. If the class
of the value FETCH()ed from the tied variable does not change, a
simple workaround for code that is to run on older Perl versions is to
access the value (via "() = $foo" or
some such) immediately after tie()ing, so that after this call
the previous class coincides with the current one.